Axial behavior of square steel-tubed ultra-high-strength reinforced concrete columns

Steel-tubed reinforced concrete (STRC) columns with ultra-high-strength concrete (UHSC; compressive strength greater than 120 MPa) are currently used in some super high-rise buildings in Japan and other Asian countries. To properly evaluate the seismic performance of steel-tubed reinforced ultra-high-strength concrete (STR-UHSC) columns, it is necessary to understand their behavior under axial compression. However, there are few studies on the axial behavior of square STRC columns, particularly with UHSC. Therefore, axial compression tests were conducted on four square STR-UHSC columns with the test variables being the amount of transverse reinforcement and the presence of the steel tube. The concrete compressive strength used was over 170 MPa. The test result showed that the presence of the steel tube and the amount of the transverse reinforcement had no significant effect on the load-bearing capacity but greatly improved the post-peak behavior. To discuss the influence of the size effect and confining effect on the concrete compressive strength, a database of 56 plain and reinforced concrete columns with UHSC, including four specimens of this study was constructed. A new formula was proposed to evaluate the concrete compressive strength under the size effect and confining effect based on the regression analysis of the database. The proposed formula well simulated the measured peak load with a proper amount of contributions from concrete and longitudinal reinforcement. Finally, axial load-axial strain relationships were simulated using the proposed concrete stress-strain relationship. The axial behavior of each specimen was well simulated by considering the size effect and confining effect of the steel tube as well as the transverse reinforcement on the ultimate strain.